Various known drive systems for hybrid vehicles including an engine and a motor as a drive source are practically implemented in accordance with widespread use of hybrid vehicles. For example, according to a known structure, an input shaft of an automated transmission is connected to an output shaft of an engine via a clutch, and a motor is connected between an output shaft of the automated transmission and driving wheels. According to the foregoing construction, a motor generator, which is a motor having an electric generating function, is most likely adopted as a motor, which allows not to be provided with a separate generator. Further, for example, an automated manual transmission (AMT), which is a manual transmission selectively engaging a set of gears among plural sets of gears and automated by an actuator. The AMT provided with a clutch at an input side excels in giving a feeling that a torque increases in accordance with an operation of an accelerator when transmitting the torque compared to a planetary gear type automated transmission provided with a torque converter at an input side.
It is preferable that the feeling that a torque increases in accordance with an operation of an accelerator when transmitting the torque is ensured during a gear shifting operation and a feeling that the vehicle is not accelerated in response to an accelerator operation is not generated even at a disconnected state of the clutch. JP2002-188716A (i.e., hereinafter referred to as Patent reference 1) discloses a power transmission apparatus for hybrid vehicle which eliminates a feeling that the vehicle is not accelerated in response to an accelerator operation during a gear shift operation. The power transmission apparatus disclosed in Patent reference 1 includes an internal combustion engine, a transmission, a clutch arranged between the internal combustion engine and the transmission, an electric motor arranged between the clutch and driving wheels, and a control portion. The control portion is configured to actuate the electric motor before disconnecting the clutch upon detecting a shift point. Thus, according to Patent reference 1, because the drive torque from the electric motor is transmitted even if the drive torque from the internal combustion engine is not transmitted due to the disconnection of the clutch, the driving wheels are not assumed to be in a completely driven state. Further, noises and vibrations because of mechanical shakiness are not generated when shifting gears, and a shift feel is enhanced because a feeling that the vehicle is not accelerated in response to an accelerator operation is eliminated during the gear shift operation.
According to the power transmission apparatus disclosed in Patent reference 1, a gear shift operation is relatively easy and effects can be readily attained in a case where the electric motor is not in operation immediately before shifting gears, however, is not completely flawless. For example, according to a hybrid vehicle to which a motor generator is mounted in place of the electric motor, in a case where a state of charge of a battery declines, the vehicle travels while generating electric power. In those circumstances, modes of the motor generator is switched from a power generation mode to an assist mode during the gear shift operation, a drive torque transmitted to the driving wheels is assumed to be discontinuous, which may generate shift shock. Further, when performing a gear shift operation in a case where the motor generator performs a drive assist by adding an assist torque of the motor generator to an engine torque for the purpose of enhancing fuel economy under a state where state of charge of the battery is favorable, the drive torque is assumed to be discontinuous so that the shift shock may be generated.
A need thus exists for a gear shift control device for a hybrid vehicle drive system which is not susceptible to the drawback mentioned above.